Most systems used in the furniture industry today to apply adhesive two-dimensionally to furniture parts, including the edge of a panel, utilize an open glue pot and roller to apply adhesive via direct contact between the roller and the panel moving past the roller. A doctor blade is typically used to control the amount of adhesive on the roller. Open systems of this type require relatively high maintenance, especially when used with polyurethane reactive adhesives since these adhesives react with moisture in the air. This causes the adhesive to cure partially and typically requires cleaning the open glue pot at the end of each shift of production.
In order to resolve problems associated with open glue pot systems, the industry has utilized “closed” systems employing a “hot melt unit” to melt the adhesive material and pump it through a heated hose at high pressure to a gun or dispensing head. The dispensing head is typically mounted on a frame, and the panel material is conveyed past the applicator head by a transport device. During operation, the adhesive is discharged through an elongated slot of a slot nozzle assembly and is dispensed onto the surface of the substrate being conveyed past the slot. The slot is usually oriented transverse to the direction of the relative motion between the dispensing head and the substrate.
Dispensing heads of the foregoing type have been successfully used in many applications, but they can exhibit certain disadvantages. For instance, it can be difficult to control the application of the hot melt material discharging from the slot, with those slots that are relatively wide. In these instances, thick layers of glue tend to go over edges of the work piece, creating cleaning issues downstream.
Another problem with the slot dispenser heads is that there is a reservoir of material remaining between the valving point and the slot after the dispenser is turned off. This volume of adhesive oozes out of the slot and can prevent the slot dispenser from finishing with a clean edge. The additional adhesive can form strings from the end of the work piece, which adds cleaning steps and potentially additional maintenance of the machinery. One approach to resolving this problem has been the use of a secondary valve and a shaft that is inserted into the. reservoir when the adhesive is dispensing and then retracted when the dispenser is turned off. The retraction of the shaft from the reservoir creates a vacuum or lower pressure, which sucks the adhesive back to provide a clean cut off of the adhesive. However, this “suck back” system adds complexity and cost to the dispenser.
Also, the use of slot type dispensing heads and glue pot roller systems can result in problems with applying adhesive to certain grades of particle board. Particle boards contain dense materials on the outside of the panel and less dense material on the inside of the panel. The lower density areas of the particle boards are more absorbent and can absorb the glue before the edge band is applied.